<html 
  window-frame="transparent" 
  window-width=420px 
  window-height=420px >
    <head>
        <title>Test</title>
        <style>

html { 
  background:transparent; 
  overflow:none;
}

body {
  width:400px;
  height:400px;
  border-radius:200px;
  margin:*;
  background:gold;
  box-shadow:2px 2px 4px #333;
  vertical-align: middle;
  text-align: center;
  transform: scale(0.1);
}

body.ready {
  transform: scale(1);
  transition: transform 400ms quad-out;
}

body > button {
  display:block;
  margin:0 *;
}

        </style>
        <script>

document.on("click", "button#close", function() {
  Window.this.close();
});

post(function() { document.body.classList.add("ready") });

        </script>
    </head>

    <body>

<div role="window-caption">
  <p>window-frame="transparent"</p>
  <p>window may have arbitrary shape</p>
</div>
<button #close>Close</button>

    </body>
</html>